C6 shifts too soon

Guys this is on the wife's 73 mach and had it rebuild in 2000 and when its cold it shifts fine but once it warms up it shifts into 2nd gear with in moving a foot when pulling away from a stop. 2nd to 3rd is fine. I have no idea if the valve body was stock when I got it back from the builder or had some kind of shift kit.

Ace

disconnect the vacuum line to the modulator and drive it , if it still shifts early it's the governor sticking
Frank

would that still be the case if it cold or hot? and what does it take to get to the governor

Ace

you have to remove tailshaft housing to get to govener .